This invention relates generally to a control system for driving an alternating current (AC) motor and more specifically relates to a multiple speed control means for a variable speed motor system. Still more specifically, the invention relates to a variable speed motor control system for controlling the vertical movement of a hoist.
The speed of a squirrel cage induction motor is proportional to the power line frequency, number of poles. and the slip of the motor. The slip of the motor is directly related to the electrical energy applied to the input power terminals of the motor. The torque of the motor is proportional to the square of the voltage applied at the input to the motor. Thus, by varying the voltage magnitude per unit of time applied at the input power terminals, the torque and the speed of the motor can be varied. In the hoist control described herein solid state thyristors are inserted between the input power source and the input power terminals of the induction motor, and the voltage to the thyristors are phase controlled. The torque and speed of the motor is varied by varying the time duration of current flow per AC cycle from the source to the motor input power terminals.
In my said prior patent application entitled "Variable Speed Motor Control System," the speed of the motor was incrementally varied from zero to maximum speed. However, many applications for such variable speed systems required multiple fixed speeds for efficient operations. The subject patent application is directed to variable speed motor control systems for controlling a hoist and includes multiple speed levels, all of which being fixed speeds except the lowest speed level which is variable throughout a predetermined speed range.
It is contemplated that the principles and the various parts of the circuity of the subject invention are suitable and adaptable for use with other type systems, particularly those systems which require control of the speed of the motor, such as, for example, cranes, conveyors, pumps, fans etc.